Posts Tagged 'algae'

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Global declines in coral reef calcium carbonate production under ocean acidification and warming

Published 11 May 2021 Science Closed
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Significance

The growth of coral reefs is threatened by the dual stressors of ocean warming and acidification. Despite a wealth of studies assessing the impacts of climate change on individual taxa, projections of their impacts on coral reef net carbonate production are limited. By projecting impacts across 233 different locations, we demonstrate that the majority of coral reefs will be unable to maintain positive net carbonate production globally by the year 2100 under representative concentration pathways RCP4.5 and 8.5, while even under RCP2.6, coral reefs will suffer reduced accretion rates. Our results provide quantitative projections of how different climate change stressors will influence whole ecosystem carbonate production across coral reefs in all major ocean basins.

Abstract

Ocean warming and acidification threaten the future growth of coral reefs. This is because the calcifying coral reef taxa that construct the calcium carbonate frameworks and cement the reef together are highly sensitive to ocean warming and acidification. However, the global-scale effects of ocean warming and acidification on rates of coral reef net carbonate production remain poorly constrained despite a wealth of studies assessing their effects on the calcification of individual organisms. Here, we present global estimates of projected future changes in coral reef net carbonate production under ocean warming and acidification. We apply a meta-analysis of responses of coral reef taxa calcification and bioerosion rates to predicted changes in coral cover driven by climate change to estimate the net carbonate production rates of 183 reefs worldwide by 2050 and 2100. We forecast mean global reef net carbonate production under representative concentration pathways (RCP) 2.6, 4.5, and 8.5 will decline by 76, 149, and 156%, respectively, by 2100. While 63% of reefs are projected to continue to accrete by 2100 under RCP2.6, 94% will be eroding by 2050 under RCP8.5, and no reefs will continue to accrete at rates matching projected sea level rise under RCP4.5 or 8.5 by 2100. Projected reduced coral cover due to bleaching events predominately drives these declines rather than the direct physiological impacts of ocean warming and acidification on calcification or bioerosion. Presently degraded reefs were also more sensitive in our analysis. These findings highlight the low likelihood that the world’s coral reefs will maintain their functional roles without near-term stabilization of atmospheric CO2 emissions.

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Acclimation history of elevated temperature reduces the tolerance of coralline algae to additional acute thermal stress

Published 10 May 2021 Science Closed
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Increasing atmospheric CO2 is driving major environmental changes in the ocean, such as an increase in average ocean temperature, a decrease in average ocean pH (ocean acidification or OA), and an increase in the number and severity of extreme climatic events (e.g., anomalous temperature events and heatwaves). Uncertainty exists in the capacity for species to withstand these stressors occurring concomitantly. Here, we tested whether an acclimation history of ocean warming (OW) and OA affects the physiological responses of an abundant, reef-building species of crustose coralline algae (CCA), Porolithon cf. onkodes, to chronic and acute thermal stress. To address this, we exposed algae to varying temperature and pH levels for 6 weeks and this chronic treatment experiment was followed by an acute exposure to an anomalous temperature event (+4–6°C from acclimation temperature). Net photosynthetic rate was negatively affected across all treatments by increasing temperature during the acute temperature event, however, algae acclimated to the control temperature were able to maintain photosynthetic rates for +4°C above their acclimation temperature, whereas algae acclimated to elevated temperature were not. Average relative change in O2 produced resulted in a 100–175% decrease, with the largest decrease found in algae acclimated to the combined treatment of elevated temperature and reduced pH. We conclude that acclimation to chronic global change stressors (i.e., OW and OA) will reduce the tolerance of P. cf. onkodes to anomalous increases in temperature, and this may have implications for reef building processes.

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Exposure time modulates the effects of climate change-related stressors on fertile sporophytes and early-life stage performance of a habitat-forming kelp species

Published 6 May 2021 Science Closed
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Highlight

  • Ocean warming (OW) reduced the sorus photosynthetic performance.
  • OW reduced meiospore germination rate.
  • OW and ocean acidification reduced meiospore release (MR).
  • MR is more sensitive to temperature changes than to pCO2 changes.
  • Longer exposure to OW increased the negative effects on germination rate.

Abstract

Understanding the impact of increases in pCO2 (OA) and extreme changes in temperature on marine organisms is critical to predicting how they will cope with climate change. We evaluated the effects of OA as well as warming and cooling trend temperature on early reproductive traits of Lessonia trabeculata, a foundation kelp species. Sori discs were maintained for an exposure time (ET) of 3 (T3) and 7 (T7) days to one of two contrasting pCO2 levels (450 and 1100 μatm). In addition, at each pCO2 level, they were subjected to three temperature treatments: 15 °C (control), 10 °C (cool) and 19 °C (warm). Subsequently, we compared sorus photosynthetic performance (Fv/Fm), the number of meiospores released (MR) and their germination rate (GR) after 48 h of settlement, with values obtained from sori discs not exposed (DNE) to the treatments. The Fv/Fm measured for DNE was lower than at T3 and T7 at 10 and 15 °C but not at 19 °C. Regardless of temperature, we found no significant differences between MR measured at T0 and T3 were found. MR at T7 was significantly lower at 19 °C than at 10 and 15 °C. We found only aA significant reduction in MR in response to elevated pCO2 was only found at T3. The GR of meiospores released by DNE and then maintained for 48 h to 19 °C decreased significantly by ∼33 % when compared with those maintained for the same time at 10 and 15 °C. A similar, but more drastic reduction (∼54 %) in the GR was found in meiospores released by sori discs exposed for T3 and maintained for 48 h to 19 °C. We suggest that OA and warming trend will threaten the early establishment of this species with further consequences for the functioning of the associated ecosystem.

Graphical abstract

Under laboratory conditions were investigated the combined effect of pCO2, temperature and the exposure time on sorus photosynthetic performance (Fv/Fm) and meiospore performance (release and germination rate) of a habitat-forming kelp Lessonia trabeculata. The results suggest that important traits such as sorus photosynthetic performance, meiospores released and germination rate can be affected by those stressors and by the extent to which the sori are exposed. We concluded that ocean warming and ocean acidification might threaten the early establishment of this species with further consequences for the ecosystem functioning, goods and services in coastal environments.

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The Bouraké semi-enclosed lagoon (New Caledonia). A natural laboratory to study the life-long adaptation of a coral reef ecosystem to climate change-like conditions

Published 4 May 2021 Science Closed
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According to current experimental evidence, coral reefs could disappear within the century if CO2 emissions remain unabated. However, recent discoveries of diverse and high cover reefs that already thrive under extreme conditions seem to contradict these projections. Volcanic CO2 vents, semi-enclosed lagoons and mangrove estuaries are unique study sites where one or more ecologically relevant parameters for life in the oceans are close or even worse than currently projected for the year 2100. These natural analogues of future conditions hold new hope for the future of coral reefs and provide unique natural laboratories to explore how reef species could keep pace with climate change. To achieve this, it is essential to characterize their environment as a whole, and accurately consider all possible environmental factors that may differ from what is expected in the future and that may possibly alter the ecosystem response.

In this study, we focus on the semi-enclosed lagoon of Bouraké (New Caledonia, SW Pacific Ocean) where a healthy reef ecosystem thrives in warm, acidified and deoxygenated water. We used a multi-scale approach to characterize the main physical-chemical parameters and mapped the benthic community composition (i.e., corals, sponges, and macroalgae). The data revealed that most physical and chemical parameters are regulated by the tide, strongly fluctuate 3 to 4 times a day, and are entirely predictable. The seawater pH and dissolved oxygen decrease during falling tide and reach extreme low values at low tide (7.2 pHT and 1.9 mg O2 L−1 at Bouraké, vs 7.9 pHT and 5.5 mg O2 L−1 at reference reefs). Dissolved oxygen, temperature, and pH fluctuates according to the tide of up to 4.91 mg O2 L−1, 6.50 °C, and 0.69 pHT units on a single day. Furthermore, the concentration of most of the chemical parameters was one- to 5-times higher at the Bouraké lagoon, particularly for organic and inorganic carbon and nitrogen, but also for some nutrients, notably silicates. Surprisingly, despite extreme environmental conditions and altered seawater chemical composition, our results reveal a diverse and high cover community of macroalgae, sponges and corals accounting for 28, 11 and 66 species, respectively. Both environmental variability and nutrient imbalance might contribute to their survival under such extreme environmental conditions. We describe the natural dynamics of the Bouraké ecosystem and its relevance as a natural laboratory to investigate the benthic organism’s adaptive responses to multiple stressors like future climate change conditions.

Continue reading ‘The Bouraké semi-enclosed lagoon (New Caledonia). A natural laboratory to study the life-long adaptation of a coral reef ecosystem to climate change-like conditions’

Tidal action and macroalgal photosynthetic activity prevent coastal acidification in an eutrophic system within a semi-desert region

Published 3 May 2021 Science Closed
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Highlights

  • Macroalgal photosynthesis (MP) controls daily pH variability during low tide.
  • Environmental factors control pH variability at seasonal scale.
  • Ulva lactuca photosynthetic activity increased the pH of seawater.
  • Macrotidal action and MP prevent coastal acidification in an eutrophic system.

Abstract

Nutrient input drive macroalgal blooms and increases in photosynthetic activity in coastal ecosystems. An intense macroalgal photosynthetic activity can increase the surrounding pH and it could prevent the acidification that often follows an eutrophication process. We tested this hypothesis with field sampling and experiments in a macrotidal (up to 9 m in amplitude) coastal system within a semi-desert region with contrasting eutrophic conditions and Ulva lactuca blooms in the northern Argentinean Patagonia (San Antonio Bay). Our results indicate that daily pH variability during low tide could be controlled by the photosynthetic activity of Ulva lactuca under eutrophic conditions. At seasonal scale, the pH variations were related to environmental features, particularly seawater temperature. Both environmental (i.e. high solar radiation, negligible freshwater inputs and, large tidal action) and anthropogenic nutrient inputs into the studied area promote the Ulva lactuca blooms, which in turn increases the surrounding pH in well oxygenated seawater through the intense photosynthetic activity. Our study shows that eutrophication instead of being a driver of acidification, could contribute to its prevention in well oxygenated marine coastal systems located within semi-desert regions.

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Seasonal photophysiological performance of adult western Baltic Fucus vesiculosus (Phaeophyceae) under ocean warming and acidification

Published 29 April 2021 Science Closed
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Shallow coastal marine ecosystems are exposed to intensive warming events in the last decade, threatening keystone macroalgal species such as the bladder wrack (Fucus vesiculosus, Phaeophyceae) in the Baltic Sea. Herein, we experimentally tested in four consecutive benthic mesocosm experiments, if the single and combined impact of elevated seawater temperature (Δ + 5°C) and pCO2 (1100 ppm) under natural irradiance conditions seasonally affected the photophysiological performance (i.e., oxygen production, in vivo chlorophyll a fluorescence, energy dissipation pathways and chlorophyll concentration) of Baltic Sea Fucus. Photosynthesis was highest in spring/early summer when water temperature and solar irradiance increases naturally, and was lowest in winter (December to January/February). Temperature had a stronger effect than pCO2 on photosynthetic performance of Fucus in all seasons. In contrast to the expectation that warmer winter conditions might be beneficial, elevated temperature conditions and sub-optimal low winter light conditions decreased photophysiological performance of Fucus. In summer, western Baltic Sea Fucus already lives close to its upper thermal tolerance limit and future warming of the Baltic Sea during summer may probably become deleterious for this species. However, our results indicate that over most of the year a combination of future ocean warming and increased pCO2 will have slightly positive effects for Fucus photophysiological performance.

Continue reading ‘Seasonal photophysiological performance of adult western Baltic Fucus vesiculosus (Phaeophyceae) under ocean warming and acidification’

Elevated CO2 influences competition for growth, photosynthetic performance and biochemical composition in Neopyropia yezoensis and Ulva prolifera

Published 28 April 2021 Science Closed
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Highlights

  • The growth of Neopyropia yezoensis was significantly inhibited by Ulva prolifera.
  • Elevated CO2 enhanced the resistance of N. yezoensis to the effect of U. prolifera.
  • Elevated CO2 increased the competitive ability of U. prolifera.

Abstract

The occurrence of various marine macroalgae in the same niche will inevitably lead to interspecific competition due to similar environmental requirements. With the increasing global atmospheric CO2 concentration, the resulting ocean acidification can potentially influence competition among macroalgae in the future. Neopyropia yezoensis (Rhodophyta, formerly Pyropia yezoensis) and the epiphytic alga Ulva prolifera (Chlorophyta) were selected for investigating competition among macroalgae grown under different CO2 conditions. The results showed that when cultured with U. prolifera, N. yezoensis‘ growth rate was significantly inhibited along with a sharp decrease in net photosynthetic rate. Although CO2 decreased the growth rate of N. yezoensis, it enhanced the resistance of the alga to the allelopathic effect of U. prolifera. While no difference was found between U. prolifera grown in monoculture and biculture, strong competitive ability was observed. CO2 could enhance this ability with higher net photosynthetic rate. However, CO2 significantly inhibited the carotenoid synthesis in both plants. This inhibition in N. yezoensis was more pronounced in the presence of U. prolifera. Biculture promoted the accumulation of soluble protein in N. yezoensis while it inhibited the process in U. prolifera. In addition, it enhanced the inhibitory effect of acidification on soluble carbohydrates of both plants. Elevated CO2 levels alleviated the competition between N. yezoensis and U. prolifera, but the latter can become the more competitive epiphytic alga which can impact the future of nori culture.

Continue reading ‘Elevated CO2 influences competition for growth, photosynthetic performance and biochemical composition in Neopyropia yezoensis and Ulva prolifera’

Elevated pCO2 reinforces preference among intertidal algae in both a specialist and generalist herbivore

Published 26 April 2021 Science Closed
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Highlights

  • Elevated pCO2 influences growth and chemical composition of some intertidal algae.
  • Herbivore preference is reinforced by resilience of preferred alga to pCO2 exposure.
  • Preference is also influenced by changes in lesser-preferred algal species.
  • Specialist and generalist feeding may be indirectly affected by ocean acidification.

Abstract

Ocean acidification (OA) can induce changes in marine organisms and species interactions. We examined OA effects on intertidal macroalgal growth, palatability, and consumption by a specialist crab (Pugettia producta) and a generalist snail (Tegula funebralis) herbivore. Moderate increases in pCO2 increased algal growth in most species, but effects of pCO2 on C:N and phenolic content varied by species. Elevated pCO2 had no effect on algal acceptability to herbivores, but did affect their preference ranks. Under elevated pCO2, electivity for a preferred kelp (Egregia menziesii) and preference rankings among algal species strengthened for both P. producta and T. funebralis, attributable to resilience of E. menziesii in elevated pCO2 and to changes in palatability among less-preferred species. Preferred algae may therefore grow more under moderate pCO2 increases in the future, but their appeal to herbivores may be strengthened by associated shifts in nutritional quality and defensive compounds in other species.

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Increased light availability modulates carbon and nitrogen accumulation in the macroalga Gracilariopsis lemaneiformis (Rhodophyta) in response to ocean acidification

Published 26 April 2021 Science Closed
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Highlights

  • The effects of light and elevated pCO2 on Gracilariopsis were examined.
  • Ocean acidification enhanced algal biomass, photosynthesis and total C/N ratios.
  • Increasing light and elevated pCO2 lowered nutritional quality of G. lemaneiformis.

Abstract

The economically important red macroalga Gracilariopsis lemaneiformis has demonstrated positive ecological functions in nutrient bioextraction efficiency and high harvestable biomass, as well as being a food and agar source owing to its richness in proteins and polysaccharides. Carbon dioxide (CO2)-induced ocean acidification has resulted in mixed nutrient compound accumulations in this marine autotroph. G. lemaneiformis also experiences light variations resulting from self-shading and varied cultivation depths. Therefore, a factorial coupling experiment was conducted to examine how growth, photosynthesis performance, soluble cell components and metabolic enzyme-driven activities respond to light availability changes and CO2 enrichment. The ocean acidification enhanced the growth characteristics, total carbon/nitrogen ratios and metabolic nutrient accumulation processes in G. lemaneiformis regardless of the light level. Photosynthetic performances, including relative electron transport rate and maximum photochemical quantum yield, were increased by high pCO2 concentrations, resulting in soluble carbohydrate accumulation. The carbon and nitrogen accumulations might result from variations in carbonic anhydrase and nitrate reductase activities under high pCO2 conditions. The soluble protein and free amino acids contents declined in response to CO2 elevation, and this effect was more pronounced as the light intensity increased. Thus, future climate changes may cause greater algal biomass accumulations, but they may negatively affect the cell composition and nutritional quality of G. lemaneiformis.

Continue reading ‘Increased light availability modulates carbon and nitrogen accumulation in the macroalga Gracilariopsis lemaneiformis (Rhodophyta) in response to ocean acidification’

An intertidal life: combined effects of acidification and winter heatwaves on a coralline alga (Ellisolandia elongata) and its associated invertebrate community

Published 20 April 2021 Science Closed
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Highlights

  • Coralline alga create a microhabitat with mitigating effect on ocean acidification
  • Temperature is the major driver of changes in the invertebrate reef community
  • Winter heatwaves and acidified conditions alter invertebrates community structure
  • Algal reef communities become dominated by opportunistic taxa

Abstract

In coastal marine ecosystems coralline algae often create biogenic reefs. These calcareous algal reefs affect their associated invertebrate communities via diurnal oscillations in photosynthesis, respiration and calcification processes. Little is known about how these biogenic reefs function and how they will be affected by climate change. We investigated the winter response of a Mediterranean intertidal biogenic reef, Ellisolandia elongate exposed in the laboratory to reduced pH conditions (i.e. ambient pH – 0.3, RCP 8.5) together with an extreme heatwave event (+1.4°C for 15 days). Response variables considered both the algal physiology (calcification and photosynthetic rates) and community structure of the associated invertebrates (at taxonomic and functional level). The combination of a reduced pH with a heatwave event caused Ellisolandia elongata to significantly increase photosynthetic activity. The high variability of calcification that occurred during simulated night time conditions, indicates that there is not a simple, linear relationship between these two and may indicate that it will resilient to future conditions of climate change.

In contrast, the associated fauna were particularly negatively affected by the heatwave event, which impoverished the communities as opportunistic taxa became dominant. Local increases in oxygen and pH driven by the algae can buffer the microhabitat in the algal fronds, thus favouring the survival of small invertebrates.

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Carbonate chemistry and temperature dynamics in an alga dominated habitat

Published 6 April 2021 Science 1 Comment
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Recent works have begun to explore the magnitude and frequency of localized changes in seawater chemistry in shallow water systems, where the effects of warming and acidification are still unpredictable. In a Mediterranean shallow coastal site, we empirically characterized the diel, seasonal, and annual pattern of pH, temperature, and associated chemical changes on one Ellisolandia elongata ‘corniche’, (i.e. a coastal algal biogenic reef). Local benthic metabolism together with temperature, were two of the main drivers for the carbonate system variation over diel and seasonal cycles. During the 12-month study, a total of seven heatwave events were recorded (two of which occurred in winter) with the longest lasting 11 days. The present study highlights the importance of improving the knowledge of changes and dynamics occurring at local scale, by extending in situ data acquisition at shallow coastal sites, in order to better assess the impacts of climate change on both environment and ecosystems.

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Experimental assessment of the impacts of ocean acidification and urchin grazing on benthic kelp forest assemblages

Published 23 March 2021 Science Closed
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Ocean acidification (OA) is likely to differentially affect the biology and physiology of calcifying and non-calcifying taxa, thereby potentially altering key ecological interactions (e.g., facilitation, competition, predation) in ways that are difficult to predict from single-species experiments. We used a two-factor experimental design to investigate how multispecies benthic assemblages in southern California kelp forests respond to OA and grazing by the purple sea urchin, Strongylocentrotus purpuratus. Settlement tiles accrued natural mixed assemblages of algae and invertebrates in a kelp forest off San Diego, CA for one year before being exposed to OA and grazing in a laboratory experiment for two months. Space occupying organisms were identified and pooled into six functional groups: calcified invertebrates, non-calcified invertebrates, calcified algae, fleshy algae, sediment, and bare space for subsequent analyses of community structure. Interestingly, communities that developed on separate tile racks were unique, despite being deployed close in space, and further changes in community structure in response to OA and grazing depended on this initial community state. On Rack 1, we found significant effects of both pCO2 and grazing with elevated pCO2 increasing cover of fleshy algae, but sea urchin grazers decreasing cover of fleshy algae. On Rack 2, we found a ~ 35% higher percent cover of sediment on tiles reared in ambient pCO2 but observed ~27% higher cover of bare space in the high pCO2 conditions. On Rack 3, we found an average of 45% lower percent cover of calcified sessile invertebrates at ambient pCO2 than in high pCO2 treatments on Rack 3. Net community calcification was 137% lower in elevated pCO2 treatments. Kelp sporophyte densities on tiles without urchins were 74% higher than on tiles with urchins and kelp densities were highest in the elevated pCO2 treatment. Urchin growth and grazing rates were 49% and 126% higher under ambient than high pCO2 conditions. This study highlights consistent negative impacts of OA on community processes such as calcification and grazing rates, even though impacts on community structure were highly context-dependent.

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Kelp aquaculture in China: a retrospective and future prospects

Published 11 March 2021 Science Closed
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Globally, China has the largest scale of kelp cultivation and production operations. However, its kelp aquaculture industry is suffering from declining germplasm diversity, degradation of agronomic traits, the presence of polluted
environments, changing ocean conditions and increasing anthropological interference. This review covers two of the most commercially important kelp species in China, viz. Saccharina japonica and Undaria pinnatifida. It summarizes the history of their cultivation, production, economic and ecological benefits, their breeding programmes (e.g. inter- and intra-specific hybridization and marker-assisted selection) and efforts towards population genetic diversity and conservation. The article focuses on three significant challenges, for example genetic crosscontamination between the wild and farmed kelp populations, ocean warming and ocean acidification. Accordingly, we outline the steps required to provide several intervention measures, for example (i) collection and preservation of wild and cultivated kelp germplasm; (ii) selection of suitable cultivation sites under changing environmental conditions; (iii) developing stress-resistant cultivars; and finally, (iv) adoption of innovative cultivation models. The review concludes with genome-based, designs for molecular breeding and calls for the establishment of an East Asian Kelp Consortium (EAKC). Collectively, the Chinese kelp industry could provide beneficial goods and services, for example bioenergy to fine chemicals and environmental benefits, such as carbon capture, pH amelioration and provision of habitat for many other marine species of commercial value. The strategies proposed in this article thus have the potential to not only improve but also reinvigorate the kelp industry in China and nearby Japan and Korea, in the context of both environmental health and economic benefits.

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Photosynthetic responses of Halimeda scabra (Chlorophyta, Bryopsidales) to interactive effects of temperature, pH, and nutrients and its carbon pathways

Published 9 March 2021 Science Closed
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In this study, we evaluated the interactive effects of temperature, pH, and nutrients on photosynthetic performance in the calcareous tropical macroalga Halimeda scabra. A significant interaction among these factors on gross photosynthesis (Pgross) was found. The highest values of Pgross were reached at the highest temperature, pH, and nutrient enrichment tested and similarly in the control treatment (no added nutrients) at 33 °C at the lowest pH. The Q10 Pgross values confirmed the effect of temperature only under nutrient enrichment scenarios. Besides the above, bicarbonate (HCO3−) absorption was assessed by the content of carbon stable isotope (δ13C) in algae tissue and by its incorporation into photosynthetic products, as well as by carbonic anhydrase (CA) inhibitors (Acetazolamide, AZ and Ethoxyzolamide, EZ) assays. The labeling of δ13C revealed this species uses both, CO2 and HCO3− forms of Ci relying on a CO2 Concentration Mechanism (CCM). These results were validated by the EZ-AZ inhibition assays in which photosynthesis inhibition was observed, indicating the action of internal CA, whereas AZ inhibitor did not affect maximum photosynthesis (Pmax). The incorporation of 13C isotope into aspartate in light and dark treatments also confirmed photosynthetic and non-photosynthetic the HCO3−uptake.

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Feedback mechanisms stabilise degraded turf algal systems at a CO2 seep site

Published 17 February 2021 Science Closed
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Human activities are rapidly changing the structure and function of coastal marine ecosystems. Large-scale replacement of kelp forests and coral reefs with turf algal mats is resulting in homogenous habitats that have less ecological and human value. Ocean acidification has strong potential to substantially favour turf algae growth, which led us to examine the mechanisms that stabilise turf algal states. Here we show that ocean acidification promotes turf algae over corals and macroalgae, mediating new habitat conditions that create stabilising feedback loops (altered physicochemical environment and microbial community, and an inhibition of recruitment) capable of locking turf systems in place. Such feedbacks help explain why degraded coastal habitats persist after being initially pushed past the tipping point by global and local anthropogenic stressors. An understanding of the mechanisms that stabilise degraded coastal habitats can be incorporated into adaptive management to better protect the contribution of coastal systems to human wellbeing.

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Ocean acidification reduces skeletal density of hardground‐forming high‐latitude crustose coralline algae

Published 15 February 2021 Science Closed
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Crustose coralline algae (CCA) function as foundation species by creating marine carbonate hardground habitats. High‐latitude species may be vulnerable to regional warming and acidification. Here, we report the results of an experiment investigating the impacts of CO2‐induced acidification (pCO2 ∼350, 490, 890, 3200 µatm) and temperature (∼6.5, 8.5, 12.5°C) on the skeletal density of two species of high‐latitude CCA: Clathromorphum compactum (CC) and C. nereostratum (CN). Skeletal density of both species significantly declined with pCO2. In CN, the density of previously deposited skeleton declined in the highest pCO2 treatment. This species was also unable to precipitate new skeleton at 12.5°C, suggesting that CN will be particularly sensitive to future warming and acidification. The decline in skeletal density exhibited by both species under future pCO2 conditions could reduce their skeletal strength, potentially rendering them more vulnerable to disturbance, and impairing their production of critical habitat in high‐latitude systems.

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The dual benefit of ocean acidification for the laminarialean kelp, Saccharina latissima: enhanced growth and reduced herbivory

Published 11 February 2021 Science Closed
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The laminarialean kelp, Saccharina latissima, is a common macroalgae along rocky shorelines that is also frequently used in aquaculture. This study examined how ocean acidification may alter the growth of S. latissima as well as grazing on S. latissima by the gastropod, Lacuna vincta. Under elevated nutrients, S. latissima experienced significantly enhanced growth at pCO2 levels >1,200 µatm compared to ambient pCO2 (~400 µatm). Elevated pCO2 (>830 µatm) also significantly reduced herbivory of L. vincta grazing on S. latissima relative to ambient pCO2. There was no difference in grazing of S. latissima previously grown under elevated or ambient pCO2, suggesting lowered herbivory was due to harm to the gastropods rather than alteration of the biochemical composition of the kelp. Decreased herbivory was specifically elicited when L. vincta were exposed to elevated pCO2 in the absence of food for >18 h prior to grazing, with reduced grazing persisting 72 h. Elevated growth of S. latissima and reduced grazing by L. vincta at 1,200 µatm pCO2 combined to increase net growth rates of S. latissima by more than four-fold relative to ambient pCO2. L. vincta consumed 70% of daily production by S. latissima under ambient pCO2 but only 38% and 9% at 800 µatm and 1,200 µatm, respectively. Collectively, decreased grazing by L. vincta coupled with enhanced growth of S. latissima under elevated pCO2 demonstrates that increased CO2 associated with climate change and/or coastal processes will dually benefit commercially and ecologically important kelps by both promoting growth and reducing grazing pressure. 

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Seaweed farms provide refugia from ocean acidification

Published 10 February 2021 Science Closed
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Seaweed farming has been proposed as a strategy for adaptation to ocean acidification, but evidence is largely lacking. Changes of pH and carbon system parameters in surface waters of three seaweed farms along a latitudinal range in China were compared, on the weeks preceding harvesting, with those of the surrounding seawaters. Results confirmed that seaweed farming is efficient in buffering acidification, with Saccharina japonica showing the highest capacity of 0.10 pH increase within the aquaculture area, followed by Gracilariopsis lemaneiformis (ΔpH = 0.04) and Porphyra haitanensis (ΔpH = 0.03). The ranges of pH variability within seaweed farms spanned 0.14-0.30 unit during the monitoring, showing intense fluctuations which may also help marine organisms adapt to enhanced pH temporal variations in the future ocean. Deficit in pCO2 in waters in seaweed farms relative to control waters averaged 58.7 ± 15.9 μatm, ranging from 27.3 to 113.9 μatm across farms. However, ΔpH did not significantly differ between day and night. Dissolved oxygen and Ωarag were also elevated in surface waters at all seaweed farms, which are benefit for the survival of calcifying organisms. Seaweed farming, which unlike natural seaweed forests, is scalable and is not dependent on suitable substrate or light availability, could serve as a low-cost adaptation strategy to ocean acidification and deoxygenation and provide important refugia from ocean acidification.

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Ocean acidification may slow the pace of tropicalization of temperate fish communities

Published 9 February 2021 Science Closed
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Poleward range extensions by warm-adapted sea urchins are switching temperate marine ecosystems from kelp-dominated to barren-dominated systems that favour the establishment of range-extending tropical fishes. Yet, such tropicalization may be buffered by ocean acidification, which reduces urchin grazing performance and the urchin barrens that tropical range-extending fishes prefer. Using ecosystems experiencing natural warming and acidification, we show that ocean acidification could buffer warming-facilitated tropicalization by reducing urchin populations (by 87%) and inhibiting the formation of barrens. This buffering effect of CO2 enrichment was observed at natural CO2 vents that are associated with a shift from a barren-dominated to a turf-dominated state, which we found is less favourable to tropical fishes. Together, these observations suggest that ocean acidification may buffer the tropicalization effect of ocean warming against urchin barren formation via multiple processes (fewer urchins and barrens) and consequently slow the increasing rate of tropicalization of temperate fish communities.

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Coastal ocean acidification and nitrogen loading facilitate invasions of the non-indigenous red macroalga, Dasysiphonia japonica

Published 4 February 2021 Science Closed
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Coastal ecosystems are prone to multiple anthropogenic and natural stressors including eutrophication, acidification, and invasive species. While the growth of some macroalgae can be promoted by excessive nutrient loading and/or elevated pCO2, responses differ among species and ecosystems. Native to the western Pacific Ocean, the filamentous, turf-forming rhodophyte, Dasysiphonia japonica, appeared in estuaries of the northeastern Atlantic Ocean during the 1980s and the northwestern Atlantic Ocean during the late 2000s. Here, we report on the southernmost expansion of the D. japonica in North America and the effects of elevated nutrients and elevated pCO2 on the growth of D. japonica over an annual cycle in Long Island, New York, USA. Growth limitation of the macroalga varied seasonally. During winter and spring, when water temperatures were < 15 °C, growth was significantly enhanced by elevated pCO2 (p < 0.05). During summer and fall, when the water temperature was 15–24 °C, growth was significantly higher under elevated nutrient treatments (p < 0.05). When temperatures reached 28 °C, the macroalga grew poorly and was unaffected by nutrients or pCO2. The δ13C content of regional populations of D. japonica was −30‰, indicating the macroalga is an obligate CO2-user. This result, coupled with significantly increased growth under elevated pCO2 when temperatures were < 15 °C, indicates this macroalga is carbon-limited during colder months, when in situ pCO2 was significantly lower in Long Island estuaries compared to warmer months when estuaries are enriched in metabolically derived CO2. The δ15N content of this macroalga (9‰) indicated it utilized wastewater-derived N and its N limitation during warmer months coincided with lower concentrations of dissolved inorganic N in the water column. Given the stimulatory effect of nutrients on this macroalga and that eutrophication can promote seasonally elevated pCO2, this study suggests that eutrophic estuaries subject to peak annual temperatures < 28 °C may be particularly vulnerable to future invasions of D. japonica as ocean acidification intensifies. Conversely, nutrient reductions would serve as a management approach that would make coastal regions more resilient to invasions by this macroalga.

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